The number of freight trains carrying oil across America has soared in the past five years, but federal officials warn that the massive steel tank cars that carry most of that oil through towns and past schools – the same cars that exploded in Quebec this summer, killing 47 -- may be unsafe and prone to rupture.

“The clock is ticking,” said Jim Arie, fire chief of the Chicago suburb of Barrington, Ill., where the number of trains that rumble across Main Street has grown from five a day to nearly one an hour. “As long as these rail cars are out there and they’re being used, potential exists for a major disaster.”

For two decades, federal officials have warned that the tank car that carries oil and ethanol, known as the DOT-111, has a serious design flaw and can split open in an accident, turning a derailment into a fiery catastrophe. At least five times since 1991, the National Transportation Safety Board (NTSB) has raised concerns about the car’s design, including its relatively thin metal skin and the possibility that cars could tear holes in each other during accidents, creating a domino effect of spills.

“If we don’t start upgrading these cars soon, my concern is that we will have a catastrophic event in the near future,” said transit safety expert John Goglia, who served on the NTSB's board from 1995 to 2004. "Even if the car doesn't crumble and get destroyed in a derailment, 66 percent of the cars are going to spill whatever product is in there, and there is some pretty hazardous material being carried around in these cars."

Yet the government has done little, allowing the industry to make some voluntary safety upgrades but leaving most of the DOT-111s in service and unaltered. Just after the Quebec crash, the agency charged with regulating tank cars, the Pipeline and Hazard Materials Safety Administration (PHMSA), put off acting for another year, though this week the agency agreed to start considering safety upgrades.

In Barrington, the people who live close to the tracks are tired of waiting.

“It’s unbelievable that something that can have this kind of impact hasn’t been addressed,” said Karen Darch, Barrington’s “president,” or mayor.

Steve Morrissey, whose window looks out on Barrington’s busy tracks, said he sleeps “with one eye open” and has prepared an emergency evacuation plan.

“I have told my family that it would not be a matter of if, but a matter of when.”

For more on oil trains, watch "NBC Nightly News with Brian Williams" tonight.

Oil train traffic is rising because the domestic oil business is booming, with production at its highest point since 1993. In a bid for freedom from foreign oil, America has stepped up ethanol production and new oil extraction methods like hydraulic fracturing, or “fracking.” North Dakota alone is now producing more than 820,000 barrels per day.

Quebec town is evacuated after a train carrying petroleum products derailed and exploded causing a massive fire.

The result: the amount of crude and ethanol traveling by rail has skyrocketed. According to figures from the Association of American Railroads (AAR), just 9,500 carloads of crude and 220,000 carloads of ethanol moved by rail in 2008. Last year the combined figure for crude and ethanol was 600,000 cars. About 70 percent of North Dakota’s oil and 70 percent of the nation’s ethanol now moves by rail.

Nearly all of the cars in the swelling fleet of mile-long oil trains crisscrossing the country are DOT-111s, which hold up to 30,000 gallons apiece.

The oil train that exploded in Lac Megantic, Quebec in July, killing 47 people and destroying half the downtown, was made up of DOT-111s. Canadian authorities are investigating the tank cars, the volatility of the oil, and the handling of the train as possible factors in the tragedy.

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Steve Morrissey of Barrington, Ill., walks on the train tracks near his house. The boom in domestic oil and ethanol means that the number of freight trains rumbling through the heart of this Chicago suburb and past the window of his house has grown from five a day to nearly one an hour.

In 2009, an ethanol train comprised of DOT-111s derailed and caught flame in Cherry Valley, Ill., 50 miles west of Barrington, killing one person and injuring nine others.

In a 2012 report on the crash, NTSB investigators again criticized the cars, concluding that the “inadequate design of the DOT-111 tank cars” played a large role in the catastrophic nature of the Cherry Valley crash, illustrating their “inability ... to withstand the forces of accidents.”

Before the report was issued, however, the industry decided to take action. In order to get out in front of any new regulations, the industry convened a task force led by the AAR to weigh revisions to DOT-111 standards.

In 2011, the task force issued voluntary standards that required newly built cars carrying crude and ethanol to have thicker steel shells, improved valve coverings, and end shields to prevent punctures – all attempts to address problems the NTSB had long highlighted. The AAR also petitioned PHMSA to adopt higher federal standards for the DOT-111.

“The industry continues to look forward to action on this request from PHMSA,” said AAR spokesperson Holly Arthur in a statement, “and in the meantime has adopted the higher tank car specifications so that all new tank cars carrying … crude oil and ethanol ordered after October of 2011 are being built to these higher standards.”

The new cars are gradually being added to the existing fleet, but most cars predate the changes. According to the AAR, about 25 percent of the approximately 32,000 cars that carry crude now meet the higher standards. Numbers for ethanol cars are less clear, but according to the Renewable Fuels Association, which represents the industry, most of the fleet of 63,000 cars is more than three years old, predating the upgrade.

The NTSB, which can only issue recommendations, not regulations, has argued that mingling the new, sturdier cars with older cars defeats the purpose of the upgrade, saying the intended safety benefits are “not realized.” The old thin-skinned cars are still vulnerable to rupture, and the newer, heavier cars can still damage them during collisions.

But the industry would prefer to replace the old-style cars by attrition. And the old-style cars can last for 40 years.

The industry has also resisted improving the existing cars via retrofitting because of technical issues.

While the industry says it's not opposed to retrofitting in principle, the AAR asserted in written comments to regulators in 2011 that retrofitting would make the cars too heavy and inhibit their movement. The group also said that derailments over the preceding five years had cost $64 million, while retrofitting all crude and ethanol DOT-111s would run $1 billion.

“There were no feasible options that could be applied to all existing and new cars,” said the AAR.

Christopher Barkan of the University of Illinois, whose extensive safety research has been used by both industry and government, agreed that making upgrades to older DOT-111s is not as simple as it may seem. “Retrofitting is the most difficult issue and it’s not just a matter of money,” Barkan said. “These cars are complicated engineering structures that move hundreds of thousands of miles and absorb all kinds of shocks and vibrations.” The effectiveness of retrofits would vary depending on the particular car and retrofit, he said, and in some cases adding extra protection and weight to existing cars may not be technically feasible.

In the wake of July’s devastating crash in Quebec, however, both the Canadian and U.S. governments are taking a fresh look at the DOT-111. The train, carrying crude from North Dakota across Canada, derailed just miles from the Maine border, and had been set to cross back into the States.

On Wednesday, the PHMSA announced that it wanted to identify “additional safety enhancements” recommended by the industry task force two years ago and then "provide the public an opportunity to comment." The process could take 18 months.

In a statement, the AAR emphasized its commitment to safety, and its record. “Freight railroads are always seeking to learn how to make their safety record ever stronger,” said spokesperson Holly Arthur. “Today, 99.997 percent of all rail hazmat shipments reach their destination without a release caused by a train accident.” Arthur also noted that the trade group had asked the government to institute tougher standards in 2011.

Former NTSB board member John Goglia said it was "pretty disheartening" that regulators "haven't addressed those problems. … [O]ur government has not risen to the occasion.”

Lisa Riordan Seville is a researcher with the NBC News investigative unit. Lisa Myers is the Senior Investigative Correspondent at NBC News.